1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive employing a velocity profile and back EMF feedback to control a voice coil motor.
2. Description of the Prior Art
FIG. 1A shows a prior art disk drive 2 comprising a disk 4 and a head 6 actuated over the disk 4 by a voice coil motor (VCM). The head 6 is connected to the distal end of an actuator arm 8 which is rotated about a pivot 10 in order to actuate the head 6 radially over the disk 4. The VCM comprises a voice coil attached to the actuator arm 8 having a first leg 12A and a second leg 12B for conducting current in a clockwise or counterclockwise direction thereby generating a magnetic flux which interacts with the magnetic field of permanent magnets (not shown) to generate a torque to rotate the actuator arm 8 about the pivot 10 in a clockwise or counterclockwise direction. The disk 4 comprises a plurality of tracks 14 defined by a plurality of embedded servo sectors 16, wherein disk drive control circuitry processes the embedded servo sectors 16 in a closed-loop servo system to seek the head 6 to a target track and maintain the head 6 over the target track during read/write operations.
When the disk drive is powered down (or otherwise idle), disk drive control circuitry 17 performs a park operation wherein the head 6 is parked and the actuator arm 8 is latched. In the embodiment of FIG. 1A, the head 6 is parked on a landing zone 18 by rotating the actuator arm 8 in the counterclockwise direction. While the head 6 is parked, the actuator arm 8 is “latched” to prevent it from rotating the head 6 away from the landing zone 18. In the embodiment of FIG. 1A, a magnet 20 attached to a crash stop 22 attracts and holds a metal tang 24 attached to the actuator arm 8. Alternatively, the actuator arm 8 may be parked on a ramp located at the outer periphery of the disk 4 using a technique known as ramp-load/ramp-unload.
FIG. 1B shows a flow diagram of the steps executed by the disk drive control circuitry 17 to unlatch the actuator arm 8 when the disk drive is powered up (or otherwise comes out of an idle state). The actuator arm 8 is typically unlatched by driving the VCM with an open-loop current since position (or velocity) information is unavailable. FIG. 1C shows a waveform illustrating the open-loop current applied to the VCM to unlatch the actuator arm 8. Referring to FIG. 1B, when an unlatch operation is initiated at step 26, an acceleration pulse is applied to the VCM at step 28 to unlatch the tang 24 from the magnet 20. The acceleration pulse comprises a current of predetermined magnitude +M applied to the VCM for a predetermined interval A (FIG. 1C). At step 30 a coast interval B allows the tang 24 to “escape” the latching force of the magnet 20. At step 32 a deceleration pulse is applied to the VCM to decelerate the actuator arm 8 to enable closed-loop position control of the actuator arm 8 by reading the embedded servo sectors 16. The deceleration pulse comprises a current of magnitude −M applied to the VCM for a predetermined interval C. The deceleration interval C is less than the acceleration interval A due to the force needed to escape the latching force of the magnet 20. The magnitude −M of the deceleration pulse is equal (or nearly equal) the magnitude +M of the acceleration pulse resulting in a “bang—bang” open-loop current profile.
Using a “bang—bang” open-loop current profile for unlatching the actuator arm 8 can generate undesirable acoustic noise for certain applications, such as digital video recorders. In addition, the “bang—bang” open-loop control may cause the head to wobble when the actuator arm unlatches thereby damaging the head 6 and or the surface of the disk 4. U.S. Pat. No. 6,097,564 teaches to estimate the velocity of the VCM from the back EMF voltage across the voice coil and to use the estimated velocity in a closed-loop system to limit the maximum velocity of the actuator arm 8 after applying an acceleration pulse. Although this technique reduces damage to the head 6 and or surface of the disk 4 during the coast interval, it does not reduce head wobble during the acceleration and deceleration intervals. Further, the '564 patent does not address the acoustic noise problem due to the “bang—bang” acceleration/deceleration pulses applied to the VCM.
U.S. Pat. No. 6,535,358 presents another problem associated with unlatching the actuator arm 8 using a “bang—bang” open-loop current profile. In the '358 patent, a magnet is embedded at least partially into a plastic crash stop, for example, using injection molding techniques. With at least part of the plastic disposed between the magnet and the tang, the maximum latching force of the magnet is reduced while maintaining sufficient latching energy. This reduces the magnitude of the acceleration pulse needed to unlatch the actuator arm 8 which reduces the maximum velocity of the actuator arm 8 during unlatch as well as the maximum torque requirements of the VCM. However, the stickiness of the plastic surface to the tang 24 can vary between disk drives as well as over time and temperature leading to inconsistent unlatch performance if nominal “bang—bang” acceleration/deceleration pulses are employed.
Acoustic noise is also a problem during the latching operation due to the tang 24 colliding with the magnet 20. The prior art latching operation typically applies a constant acceleration current to the VCM to accelerate the actuator arm 8 until it latches. As the tang 24 approaches the magnet 20, the exponential increase in the force of the magnet 20 and the corresponding increase in the acceleration of the actuator arm 8 further exacerbates the acoustic noise when the tang 24 collides with the magnet 20.
There is, therefore, a need to improve the unlatching/latching or ramp-loading/ramp-unloading operations in a disk drive.